What is the mixing process?

Mixing is a fundamental radio frequency process used in heterodyne receivers and transmitters. It involves combining two different input frequencies in a non-linear device, such as a diode or transistor, to create new frequencies. The primary useful outputs of this process are the sum of the two input frequencies and the difference between them. The other options describe entirely different processes. Recovering intelligence from a modulated signal is called demodulation or detection, not mixing. Eliminating noise by phase comparison or differentiation relates to specific noise reduction or FM detection techniques, entirely separate from the core sum-and-difference mixing process.